I. Field of the Invention
The present invention relates to a relatively high-power plastic encapsulated semiconductor device and a method for manufacturing the same.
II. Description of the Prior Art
Plastic encapsulated semiconductor devices are superior to metal encapsulated semiconductor devices in ease of mass production and manufacturing costs. However, the plastic encapsulated semiconductor devices are inferior to the metal encapsulated semiconductor devices in radiation of heat when they are operated. Plastic encapsulation of semiconductor devices has recently been developed. A high power transistor manufactured by plastic encapsulation has been proposed. In this case, sufficient consideration is taken to allow for the radiation of heat.
In a transistor adhered on a metal substrate support and encapsulated by plastic, for example, the lower surface of the substrate support is not covered with plastic but exposed. The substrate support is mounted on a radiator to radiate heat. However, in this case, the substrate support must be electrically insulated from the radiator. The mounting operation of the semiconductor device and the radiator through an insulating plate is complicated and cumbersome.
On the other hand, a plastic encapsulated power transistor is proposed wherein a thin plastic layer is formed on the lower surface of the substrate support during plastic encapsulation and an insulating plate is not required for mounting the power transistor on the radiator. However, in this case, at the time of plastic encapsulation, only the side ot the lead frame from which extend the external lead is clamped by the upper and lower molds with a transistor assembly which has the external lead on one side. Plastic is injected while the substrate support is floating in a cavity defined by the molds. Thus, the substrate support may be bent in the cavity due to the injection pressure of the plastic. As a result, it is very difficult to encapsulate in plastic while keeping the substrate support in a proper position, thus, resulting in non-uniformity in the thickness of the plastic layer on the lower surface of the substrate support and degrading radiation characteristics.
The present applicant proposed a method for manufacturing a plastic encapsulated semiconductor device in Japanese Patent Application No. 32229/1981 (corresponding to U.S. patent application Ser. No. 06/352,119, filed Feb. 25, 1982) wherein a semiconductor device assembly is formed using a lead frame which has an external lead extending from one side of a substrate support and strips extending from the opposite side of the substrate support; and the external lead and the strips are clamped by upper and lower molds so as to float the substrate support in a cavity defined by the upper and lower molds during plastic encapsulation. According to this method, the substrate support is properly held and may not be bent regardless of the injection pressure of the plastic. Therefore, the thickness of a thin plastic layer formed on the rear surface of the substrate support may be made uniform and may be controlled with high precision.
However, the cut surfaces of the strips are exposed on the side surface of the plastic encapsulating housing of the semiconductor device. When this semiconductor device is mounted in electrical equipment, the cut surfaces may come into contact with other components, resulting in short-circuiting. The semiconductor device of this type has therefore required further improvement.